The snow forecast map shows the forecast snowfall in inches for the next 48 hours. Snow is a type of precipitation in the form of crystalline water ice, consisting of a multitude of snowflakes that fall from clouds. Since snow is composed of small ice particles, it is a granular material. It has an open and therefore soft structure, unless packed by external pressure. The METAR code for snow is SN.
Snow forms when water vapor condenses directly into ice crystals, usually in a cloud. Floating cloud particles (ice nucleators, often of biological origin) are needed in order for snowflakes to form at temperatures above -40C. 85% of these nuclei are airborne bacteria, with dust particles making up the rest. The ice crystals which form around the ice nucleators typically have a diameter of several millimeters and usually have six lines of symmetry. A snowflake is an aggregate of such ice crystals and may be several centimeters large. The term "snowflake" is also used below for the symmetrical ice crystals themselves. The individual ice crystals are clear but because of the amount of light the individual crystals reflect snowflakes appear white in color unless contaminated by impurities.